This invention relates generally to actuating screws and more particularly to a holddown lock screw as is commonly used in pressurized wellhead structures for such functions as holding down a casing or tubing hanger mounted in the wellhead or for energizing a pack-off seal or any of a variety of wellhead seal devices.
In a typical wellhead structure, one or more concentrically disposed casing strings are suspended from casing hangers which are supported within the bore of a casing head member. A hanger bowl lowered into the wellhead provides support for a tubing hanger which receives one or more tubing strings extending down into the well within the innermost casing string.
The casing hangers and tubing hanger are normally held down by a plurality of holddown screws which are threaded into the wellhead and against camming surfaces of the hangers in order to urge these components downwardly. The screws are customarily arranged in equiangular spacing in co-planar dispostion about the wellhead and are adapted to be moved axially in a radial direction of the longitudinal axis of the wellhead. The holddown screws thus act to prevent the blowout of these wellhead components. In many other applications holddown screws are used to provide a preload for energizing pack-offs or other wellhead seal devices. Typically, the nose end of each of the screws engages a camming surface of the seal device or pack-off to apply an energizing force in either the upward or downward direction depending upon the particular type of seal or pack-off device. The nose end of the holddown screw is customarily of conical configuration and is adapted to engage a frusto-conical surface of the component to be held down or engergized. The shank and nose of the holddown screw are conventionally a single piece. Rotation of the screw which drives it inwardly against the matting surface of the wellhead component causes the conical nose of the screw to rotate thereagainst and the associated large stresses at their line of contact makes either of the contacting surfaces very susceptible to galling.
It is therefore a primary object of the invention to provide a holddown screw design which will minimize or reduce the susceptibility to galling of the holddown screw or the surface of the object to be actuated.
Another object of the invention is to provide a holddown screw, the functioning of which is associated with lesser contact stresses between the screw and the actuated object than has heretofore been the case.
A further object is to provide a holddown screw suitable for wellhead applications wherein the screw is provided with a contact nose which will not rotate against the mating camming surface of the object to actuated or energized.